1. Technical Field
This disclosure relates generally to electronic circuits, and more particularly to a circuit and method for generating a clock signal, as well as a real time clock device comprising the circuit for generating a clock signal.
2. Description of the Related Art
A crystal oscillator is an electronic circuit that uses the mechanical resonance of a vibrating crystal of piezoelectric material to create an electrical signal with a very precise frequency. This frequency is commonly used to keep track of time (as in quartz wristwatches), to provide a stable clock signal for digital integrated circuits, and to stabilize frequencies for radio transmitters and receivers. The most common type of piezoelectric resonator is the quartz crystal, so oscillator circuits based on the crystal are called “crystal oscillators”.
Every quartz crystal has a unique resonant frequency that depends on the specifics of its manufacturing, for example, the cut angle of its crystal blank. The greater the angle error, the greater the difference between the resonant frequency and the nominal frequency will be. Furthermore, environmental changes of temperature, humidity, pressure, and vibration can also change the resonant frequency of a quartz crystal, and may lead to degraded accuracy of the time information or clock signal in systems which, for example, may include a real time clock (RTC) device, a communication terminal and so on.
One approach that has been used to compensate for the time or signal error is adjusting the frequency at which the crystal oscillator oscillates by adding additional capacitive load across a crystal. Another approach that has been used is periodically modifying the frequency by deleting or inserting clock pulses into the clock signal. These approaches result in an accuracy that is not much better than 3-5 ppm, that is, 3-5 μs of error.
Thus, there is a need for a circuit and a method for generating a clock signal with higher accuracy.